Recently, a trend toward size reduction of electronic and electric apparatus has become more pronounced, along with demand for enhancing integration of LSIs, which are employed to control functions of these apparatus. In other words, electric circuits as well as each and every electronic element must have a higher degree of integration.
In the course of enhancing integration, precious metals such as ruthenium or iridium have been employed as materials which are more useful for providing an electrode to a wafer of semiconductor devices, since precious metals provide a thin-film electrode having excellent electrode properties.
A variety of methods, such as vacuum vapor deposition and CVD, have been employed for forming thin film contained in semiconductor devices. Of these methods, sputtering, which is one type of physical vapor deposition method, is most widely employed at present. Sputtering is a method for forming a metal thin film, and comprises the steps of causing particles such as argon ions to collide with a target constituting the material of a thin film to be produced, and depositing metal particles on a substrate released by exchange of momentum. Therefore, since the properties of the formed thin film are apt to vary depending on characteristics of the target material, such as purity, high purity is a critical requirement for a target material.
A precious metal target material has conventionally been produced through either one of the following typical methods; i.e., powder metallurgy, in which precious metal powder is shaped and sintered through hot-pressing (HIP); and a casting method in which a compact of ruthenium powder formed by hot-pressing is melted in a crucible by means of irradiation with an electron beam and solidified.
In the case of the casting method, the purity of a target material is easily controlled, to thereby obtain a target material of comparatively high purity. However, much energy is required for melting a precious metal having a high melting point, and during casting a raw material must be provided in an amount greater than that found in actual products. In addition, the method has a drawback in that the number of production steps is high, to thereby elevate production cost and product price.
Powder metallurgy can produce a sputtering target material at lower energy cost than can casting. Furthermore, powder metallurgy has an advantage of providing high yield. However, a binder cannot be used during production of a sputtering target material through powder metallurgy, since the material must have high purity. Thus, a metal powder constituting a sputtering target material must be sintered and solidified without use of a binder. Without use of a binder, appropriately shaping and sintering a metal powder is very difficult, as is determining parameters for several process conditions.
Even though a powder can be sintered without use of a binder, a raw material powder easily becomes contaminated or easily adsorbs impurities, and very careful storage of the raw material for producing a sputtering target material is required. Thus, when a sputtering target material is produced through powder metallurgy, it is difficult to produce a target material of uniform structure and purity which permits use of the target for producing an electronic element in the electronic industry, unless production conditions, including storage of raw material powder, are very strictly controlled. In addition, production through powder metallurgy comprises cumbersome steps, such as producing a raw material powder and hot-pressing. These steps increase production cost, to thereby disadvantageously elevate product price.
As described above, although several practical methods for producing a precious metal target material have been developed, these methods are not necessarily satisfactory in view of product properties and production cost. Thus, demand exists for a more efficient method for producing a precious metal target material.